With the development of medical technology, medical scanning is increasingly becoming an important diagnostic and therapeutic tool in many medical applications. For example, computed tomography (CT) has been widely used in the diagnosis and radiation treatment of patients. In CT systems, a fan-shaped beam projected by an X-ray source is calibrated (or collimated) to a X-Y plane, namely, “an imaging plane”, of a Cartesian coordinate system. The X-ray beam penetrates the target object to be imaged (such as a patient) and arrives at a detector array after being attenuated by different parts of the target object. The attenuations of the X-ray beam are detected by the detector array so that an X-ray image (i.e., a CT image) can be formed.
Recently, increased attention and endeavor have been paid to reduce unnecessary radiation dose delivered to a patient. A commonly used method to reduce dose is to reduce the thickness of the X-ray beam in the longitudinal direction (namely, the Z-axis direction of the coordinate system of the CT system). Current CT manufacturers usually use a pre-patient collimator for extra X-ray beam obstruction. In the state of art, in order to accommodate the different requirements on the beam width in the Z-axis direction, two kinds of collimators have been designed. One kind of collimator has two aperture edges which can move independently to provide apertures of different widths. The other kind of collimator comprises one or more fixed width apertures.
The rotating gantry of the third generation CT systems introduces Z motion at different rotation angles. Since the X-ray tube is mounted rigidly on the gantry, the focal spot Z motion is introduced as the gantry rotates. The focal spot motion due to thermal expansion is another source of Z motion. On one hand, the collimator needs to have a narrow aperture for optimal dose reduction; on the other hand, a wide aperture is desired to accommodate the focal spot movement along the Z direction.
To overcome this contradiction, Toth et al proposed a method to dynamically track the motion of the focal spot of the X-ray source using Z ratio (“A dose reduction x-ray beam positioning system for high-speed multislice CT scanners”, Med. Phys. Vol. 27, No. 12, December 2000). The contents of this article are incorporated by reference in its entirety into this application.
For other prior art technologies on focal spot tracking, reference can be made to U.S. Pat. No. 5,644,614, U.S. Pat. No. 7,317,786, U.S. Pat. No. 5,469,429, U.S. Pat. No. 5,299,250, and so on. The contents of these patents are also incorporated by reference in their entirety into this application.
Although these prior art technologies have alleviated the contradiction on the size of the collimator aperture to some extent, there are such problems as the existence of relatively large mutual interference between the aperture edges and failure to provide adequate tracking operation range in the use of prior art collimators with fixed aperture width. Especially for a fixed width sub-millimeter aperture, these problems are more serious.
In addition, existing collimators also have drawbacks in other aspects, such as poor tracking sensitivity and poor resistance to noise and other fluctuations. Therefore, there is a need for a solution to improve one or more aspects of the existing technology, thereby reducing mutual interference between the aperture edges in beam tracking, increasing tracking range, improving tracking sensitivity, enhancing anti-noise ability, or the like.